This invention relates to a videodisc playback system that reproduces information signals such as video and audio signals from a videodisc. The purpose of this invention is to produce a videodisc playback system designed such that even if there are omissions in parts of the signals recorded on the videodisc, they do not stop the progression of the reproduced images during playback time, and a still image does not occur.
A videodisc playback system is generally equipped with a signal reader having a means to mount and rotate a videodisc where information signals are recorded as a form of longitudinally positioned indentations forming spiral tracks; a converting device for detecting and reporducing information signals, etc.; a shifting device that shifts the scanning position of the said converting device in a direction that intersects the above-mentioned tracks almost at right angles, that is, in the radial direction of the videodisc; and a control device that controls the above-mentioned shifting device so that the above-mentioned scanning position is on the track.
Videodisc playback systems are available as optical, capacitance, or mechanical playback systems. Using an optical videodisc playback system as an example, the outline of its signal reader unit is briefly explained regarding the part related to this invention.
A videodisc with recorded signals to be reproduced by an optical videodisc playback system is structured as shown in FIG. 1. On the videodisc (simply noted as disc hereafter) 1, FM-modulated signals that are multiplexed with a video signal and an audio signal are recorded as longitudinally positioned indentations forming spiral tracks as shown by track 2. The above-mentioned indented area is called a pit and is shown as 5 in FIG. 1. For example, when the disc 1 is rotated at a uniform angular velocity, the video signals are recorded so that 1/2 revolution of the disc 1 comprises one field, and one revolution makes one frame. The vertical blanking time in the composite video signal, that is, the vertical retrace time is positioned at areas 3 and 4 in FIG. 1. Usually, key code signals to discriminate the field, frame, or track are recorded in the vertical retrace time. As for the recording of information on the disc 1, the track pitch is 1.5-2 micrometers, and the pit length, which varies from the outer circumference side to the inner circumference side of the disc 1, ranges from 0.5-2 micrometers resulting in the very high density recording of signals.
As shown in FIG. 2, the videodisc playback system is equipped with a motor for rotating the removable disc 1, which is affixed on a turntable 6, at a constant angular velocity and a control circuit 7 to control the revolutions of the said motor. The videodisc playback is equipped with an optical system 8 for reading the signals from the recorded information surface of the disc 1 rotated by the motor. As shown in FIG. 3, the optical system 8 is equipped with a laser 11 that emits a linearly polarized light flux; a raster grating 12 for obtaining three light beams from the laser 11; a spot lens 13; a beam-splitting polarizing prism 14 for reflecting the light flux passed through the spot lens 13; a 1/4 wavelength plate 15 that converts the light reflected by the prism 14 to circular polarized light flux; a reflecting mirror (noted as tangential mirror) 16 for total reflection of the circular polarized light flux from the 1/4 wave length plate 15 and deflection in a direction tangential to the track; a reflecting mirror (noted as tracking mirror) 17 that deflects the light from the tangential mirror 16 in the radial direction of the disc 1; and an objective lens 18 which focuses the beam from the laser 11 onto the recorded signal surface of the disc 1. This beam is modulated at pits 5, returns in a path reverse to that taken above, converted to a linear polarized wave with the 1/4 wavelength plate 15, passed through the beam-splitting polarizing prism 14 and a cylindrical lens 19, and the reflected beam from the disc 1 is projected onto a photoelectric conversion system 20 and is read as an electrical signal.
The optical system 8 is shifted in the radial direction of the disc 1 by a shifting device 9 that is driven by a shifting motor 10. In order to read the signals correctly from the disc 1, it is necessary to rotate the disc 1 at a constant angular velocity, and the motor that rotates the disc 1 is controlled by a servo system. In addition, related also to optical system 8, it is necessary to focus the beam on the recorded signal surface on the disc 1 as mentioned before. Consequently, focus controlling is done from the surface of the disc 1 to control the position of the objective lens 18. Tangential control is also done by controlling the angle of the tangential mirror 16 and driving the light spot on the disc 1 in a direction tangential to the tracks 2 to suppress time-base fluctuation. Furthermore, tracking control is done in which the angle of the tracking mirror 17 is controlled so that the light spot scans over the center of the width of the tracks 2, and shifting of the optical system 8 is controlled. Of these control systems, the control of revolutions, focus control, and tangential control are not directly related to this invention, and are not discussed any further.
Tracking control is used, as shown in FIG. 4(a), to generate tracking light beams 22 and 23 with the signal-reading light beam 21 (called playback light beam hereafter) in between. The tracking light beams 22 and 23 scan over the disc 1 in synchronism with the playback light beam 21 to detect whether or not the playback light beam 21 is correctly scanning over the tracks. The method shown here is called a triple beam system, and the tracking light beams 22 and 23 are formed by the raster grating 12. There is another method called a single beam method. In this case, the diffracted light, which is generated when the light beam strikes the pits, is used to detect tracking.
The playback light beam 21 and the tracking light beams 22 and 23 are in a prescribed relationship of trigonometric positions: when the playback light beam 21 is in the normal position, the tracking light beams 22 and 23 are not centrally positioned in the track width, but they are set, as shown in FIG. 4(a), such that the tracking light beam 22 that precedes the playback light beam 21 is positioned off to the lower side of pits 5 in FIG. 4(a), the tracking light beam 23 that follows is positioned off to the upper side of pits 5 in FIG. 4(a), and the areas on pits 5 affected by tracking light beams 22 and 23 are equivalent. Consequently, when the playback light beam 21 is positioned off the tracks 2, the areas on pits 5 affected by tracking light beams 22 and 23 will be different. In order to have the playback light beam 21 scan the center of the tracks 2 by applying the above fact, the reflected beams of tracking light beams 22 and 23 are detected by photoelectric converter elements 25 and 26 respectively as shown in FIG. 4(b), and the outputs of the photoelectric converter elements 25 and 26 are inputted into a differential amplifier 27 and their difference is detected and amplified to determine whether or not the playback light beam 21 is correctly positioned over the track 2. The reflected beam of the playback light beam 21 is received by the photoelectric converter element 24, converted into electrical signals, and derived as a playback RF output. The output of the differential amplifier 27 is a tracking error signal. The output signal of the differential amplifier 27, that is the tracking error signal, drives the tracking mirror 17 through the driver amplifier 28 and concurrently drives the shifting motor 10 through the driver amplifier 29 so that the tracking error signal is controlled to zero.
There are cases when pits 5 on disc 1 are missed due to scratches or dust adhesion on the disc 1 during the manufacturing process or in storage after they were manufactured. When pits 5 are missed, a change in tracking error voltage cannot be obtained, and in spite of the fact that tracks to be played back are being shifted in the radial direction of disc 1 by the rotation of disc 1, the playback light beam and tracking light beam maintain the position just before the missing pit for a short time. Consequently, the tracking light beam will have shifted to the side of the neighboring track during the missing pit period. The signal recorded on this track to which shifting occurred, is a signal which had already been read, and this phenomenon is repeated every time the missing pit is encountered. Thus, the playback images stop progressing, the playback shifting is interrupted, and a still image results.
The above explanation was made regarding the case of optical videodisc playback system. However, the same phenomenon is induced in the case of a capacitance or mechanical videodisc playback system where the principle is the same as with the above-mentioned system.